linux-kernel - Re: [RFC PATCH 0/3] mm: memcontrol: delayed force empty

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Message-ID: <88b4d986-0b3c-cbf0-65ad-95f3e8ccd870@linux.alibaba.com>
Date:   Thu, 3 Jan 2019 11:49:32 -0800
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     hannes@...xchg.org, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] mm: memcontrol: delayed force empty



On 1/3/19 11:23 AM, Michal Hocko wrote:
> On Thu 03-01-19 11:10:00, Yang Shi wrote:
>>
>> On 1/3/19 10:53 AM, Michal Hocko wrote:
>>> On Thu 03-01-19 10:40:54, Yang Shi wrote:
>>>> On 1/3/19 10:13 AM, Michal Hocko wrote:
> [...]
>>>>> Is there any reason for your scripts to be strictly sequential here? In
>>>>> other words why cannot you offload those expensive operations to a
>>>>> detached context in _userspace_?
>>>> I would say it has not to be strictly sequential. The above script is just
>>>> an example to illustrate the pattern. But, sometimes it may hit such pattern
>>>> due to the complicated cluster scheduling and container scheduling in the
>>>> production environment, for example the creation process might be scheduled
>>>> to the same CPU which is doing force_empty. I have to say I don't know too
>>>> much about the internals of the container scheduling.
>>> In that case I do not see a strong reason to implement the offloding
>>> into the kernel. It is an additional code and semantic to maintain.
>> Yes, it does introduce some additional code and semantic, but IMHO, it is
>> quite simple and very straight forward, isn't it? Just utilize the existing
>> css offline worker. And, that a couple of lines of code do improve some
>> throughput issues for some real usecases.
> I do not really care it is few LOC. It is more important that it is
> conflating force_empty into offlining logic. There was a good reason to
> remove reparenting/emptying the memcg during the offline. Considering
> that you can offload force_empty from userspace trivially then I do not
> see any reason to implement it in the kernel.

Er, I may not articulate in the earlier email, force_empty can not be 
offloaded from userspace *trivially*. IOWs the container scheduler may 
unexpectedly overcommit something due to the stall of synchronous force 
empty, which can't be figured out by userspace before it actually 
happens. The scheduler doesn't know how long force_empty would take. If 
the force_empty could be offloaded by kernel, it would make scheduler's 
life much easier. This is not something userspace could do.

>
>>> I think it is more important to discuss whether we want to introduce
>>> force_empty in cgroup v2.
>> We would prefer have it in v2 as well.
> Then bring this up in a separate email thread please.

Sure. Will prepare the patches later.

Thanks,
Yang